It is well known to electrical utilities and their industrial customers that alternating current (AC) electrical systems commonly have a reactive load, i.e. a load having an inductive or capacitive component. Most commonly the reactive load is inductive. Due to the effects of these loads and the elements of the electrical system, there is a difference between the real power available to the load and the apparent power supplied by the source. This difference is referred to as reactive power, measured in volt-amperes reactive or VAR. The power factor (PF) is the ratio of real power (W) over reactive power (VAR). It is preferable to maintain the power factor as close to one as possible. An inductive load causes a lagging reactive current since the voltage leads the current, and a capacitive load causes a leading current since the current leads the voltage.
Leading PF loads are caused by elements such as fluorescent lightbulbs; however, most industrial installations use significant numbers of motors or equipment, such as air conditioners, which cause inductive loads. This has a significant effect on the power factor for industrial users where it is known to have inefficient power factors as low as 0.6. This is undesirable from the utility or source's standpoint because the utility must supply the apparent power while only being able to charge for the real power percentage. Additionally, the excess, unused reactive power on the line causes excess losses and heat requiring larger equipment or more generation. Other effects may include poor voltage regulation at transformers or false signals to overvoltage regulation devices.
To encourage industries to maintain a power factor approaching one (unity), utilities often charge industries a premium for power supplied when the power factor falls below a set level such as 0.9. As such, it is also in the industries' best interest to maintain a power factor near unity. A near unity power factor also allows more constant and efficient transmission of power over smaller lines or allows more power to be distributed over the lines. Although possible for residential installations, normally residential users do not carry sufficient inductive loads to justify power factor compensation.
Capacitors installed at the industrial user's location are an efficient way to compensate for lagging current but traditionally have been subject to certain disadvantages. In particular, the compensation of a capacitor cannot be varied over time, while the inductive load may vary. Accordingly, the capacitor may be insufficient for a varied demand, or may provide excess correction leading to an overvoltage situation. Utility company or power engineers in the past have calculated the correction for an industrial user and installed single or multiple capacitors, but have not had the ability to adjust the capacitance in response to drastic changes in power factors or operation outside of the designed range.
Recently, the ability to dynamically add and subtract capacitors or capacitor banks has been envisioned using a microprocessor type of controller. For example, U.S. Pat. No. 5,670,864 to Marx describes a controller which measures the phase angle variation and adaptively connects or disconnects capacitor banks. U.S. Pat. No. 5,469,045 to Dove illustrates a high speed microprocessor which senses current and voltage and corrects the power factor by connecting and disconnecting capacitor banks.
Although these designs address some of the problems in correcting power factor, they do not solve all of the problems. In particular, a resonance problem can arise in a power system when a controller switches a capacitor into or out of a system. When a capacitor is switched, normally there is a ringing transient. This ringing transient should attenuate in a few cycles unless the frequency is the same as a harmonic generated by the customer thereby producing resonance which results in a high harmonic level. The percentage of harmonic energy or distortion is called the Total Harmonic Distortion (THD). If the THD is too high and remains that way, there can be damage to the system such as overload of the capacitor banks or insulation damage to the customers' equipment. Electrical customers are required to ensure that the THD does not exceed five percent (5%) due to their load. Utilities may charge customers a premium or, in a worst case, disconnect a customer if the THD is too high.
Normally a study is carried out to size the capacitor banks to the installation in order to minimize the probability of resonance. Unfortunately this model may be inaccurate, or changes may be made to the system which result in errors in the model and the potential for damage. Additionally the model becomes more complex when capacitor banks are dynamically switched into or out of the system. Accordingly, there is a need for an apparatus which can be used in combination with a capacitor bank switching controller, which minimizes or prevents electrical harmonic resonance thereby preventing high levels of THD due to resonance in an electrical system after a capacitor bank is switched.